This invention relates to a process for the solvent refining of coal wherein coal is liquefied by subjecting it to a hydrogen donor solvent (hereinafter referred to as "solvent") in the presence of a hydrogen-rich gas at elevated temperatures and pressures to produce solid and liquid products. This process is referred to in the art as SRC-I, solvent refined coal having the acronym "SRC".
In this process, following solvation, the products are separated into gaseous material, distillate fractions and vacuum distillation bottoms. The vacuum distillation bottoms, which contain entrained mineral matter and unconverted coal macerals, are separated in a deashing step. From the solids removal step there is recovered a stream of coal products which are free of ash minerals and unconverted coal and which are essentially low in sulfur content, such that this material is ideally suited for combustion in environmentally acceptable operations.
The SRC-I pilot plants at Wilsonville, Ala. and Fort Lewis, Wash. have been operated with a coal liquefaction reactor (also known as the dissolver) preceded by a preheater. The coal liquefaction reactions take place to some extent in both these vessels. A slurry of coal in recycled solvent under hydrogen pressure is passed through the preheater where its temperature is raised from ambient to a temperature in excess of 750.degree. F. (398.9.degree. C.). The heated slurry is passed to the reactor wherein the reaction of the hydrogen gas, the coal and the solvent take place at temperatures in excess of 780.degree. F. (415.6.degree. C.) and pressures in excess of 1,000 psia (70.3 kg/cm.sup.2 a), the liquefaction reactions including desulfurization, solvent production, solvent rehydrogenation, etc. So long as hydrogen gas is present, the forward rate of reaction to produce asphaltenes and oils from dissolved coal is greater than the retrograde repolymerizations which lead to the formation of coke and preasphaltenes from the lower molecular products.
It is desirable to use a coal liquefaction reactor comprising at least two reactors (dissolvers) in series in order to maximize the ratio of asphaltene to preasphaltene in the product SRC. Also, while it is desirable to operate both reactors at the same temperature, the heat of reaction causes the second reactor in the series to operate at a higher temperature unless cooling is applied to the second reactor.
The conventional method of cooling in processes for the direct liquefaction of coal is by the addition of cold recycled hydrogen gas. A problem with this approach when applied in the SRC-I process is that the quantity of hydrogen required chemically for addition at the second reactor can only provide cooling equivalent of about 20.degree. F. (11.1.degree. C.), even when the hydrogen stream has been cooled to as low as 200.degree. F. (93.3.degree. C.). In a typical case, a cooling quantity of about 50.degree. F. (27.8.degree. C.) is required to achieve equal temperatures in the series reactors operating in the range of 825.degree.-850.degree. F. (440.6.degree.-454.4.degree. C.). Accordingly, this conventional method of cooling requires an extensive quantity of hydrogen resulting in high recycle equipment cost and high energy usage.
A second conventional method of cooling is by the direct cooling of the slurry flowing from the first reactor to the second reactor with a heat exchanger. This second method requires the use of very complex and expensive apparatus for achieving heat exchange between a high pressure three-phase gas-liquid-solid (hydrogen plus undissolved coal slurry) system and a suitable coolant (for example, cold recycle solvent).